Project Summary/Abstract LINE-1 (L1) retrotransposons are active elements in the genome capable of mobilization in neuronal precursor cells, resulting in a mosaic in the brain. Upon mobilization, L1 insertions can alter gene expression, resulting in a genetic heterogenic population of neurons. The first aim of this proposal is to identify genomic loci of L1s in the human brain as compared to somatic tissues from the same individual in order to map and characterize de novo insertions in the brain. The second aim is to attenuate L1 retrotransposition in a mouse model and thereby investigate the functional significance of L1 in behavior.